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The US refusal to condemn the Israeli raid on the Gaza ships angered Turks

It's been a difficult few weeks for US-Turkish relations.

First there was the nuclear deal with Iran mediated by Turkey and Brazil last month. It was meant to build confidence but irked Washington.

Then the Obama administration stopped short of condemning Israel's raid against a Turkish ship heading to Gaza, upsetting Ankara.

After that came Turkey's vote at the UN Security Council on sanctions against Iran - a "No" vote on an issue of key strategic interest to the US.

Newspapers in Turkey described it as a turning point in ties between countries that have been allies for decades.

Washington 'disappointed'

In an interview with the BBC, US State Department official Phil Gordon said Washington was "disappointed that (Turkey) didn't stand with the United States as a longstanding Nato ally".

The assistant secretary of state for Europe and Eurasia said he did not doubt that Turkey was sincere in wanting to work with the international community to prevent Iran from pursuing a nuclear programme.

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Turkey voted against a US-backed resolution on tougher sanctions on Iran

But he added Turkey clearly had different views about the sanctions resolution.

Despite the disappointment, US officials have tried to sound sanguine about the disagreements with Turkey.

"Turkey and the United States have never been without their differences, and we have some important differences now, but we also have a lot in common that we're working together on," said Mr Gordon.

Foreign policy evolving

However, the tension raised questions in Washington about whether the US is losing Turkey as an ally.

While the relationship may be changing, so far no one either here or in Turkey is worried about a breakdown. But everybody is closely watching Turkey's evolving foreign policy.

Defence Secretary Robert Gates made comments in London that suggested Turkey was turning away from the West.

While he did not quite state it as an established fact, he did make clear why he thought Ankara could be going in that direction.

"I personally think that if there is anything to the notion that Turkey is, if you will, moving eastward," said Mr Gates, "it is, in my view, in no small part because it was pushed, and pushed by some in Europe refusing to give Turkey the kind of organic link to the West that Turkey sought."

If this is the view in Washington, then it is likely the Obama administration will be careful not to act in a way that could further push Turkey towards the Arab and Islamic world.

The US also needs Turkey in places like Iraq and Afghanistan.

Turkey extending ties

Turkey has also rejected the notion that it is looking more East with such a vehemence it seems to suggest it is worried about sending the wrong signals to the West.

We had no choice but to vote like thisNuh Yilmaz, Seta (Turkish think-tank)

Turkey is simply trying to "diversify its relations," says Nuh Yilmaz, the Washington DC director of the Turkish political think-tank Seta.

It is pursuing ties with countries with which it had limited or bad contact before, he said. Turkey's vote at the UN was not about embracing Iran even if that was the impression it left.

"We had no choice but to vote like this," said Mr Yilmaz. "Not because Iran is right, but because we had to stand by the deal that we reached with Iran, together with Brazil."

Strategic role

But there is little doubt that Ankara is slowly becoming a more assertive regional player. And that could make it a strategic competitor to the US.

"Turkey is no doubt playing a more active role in the region," said Mr Gordon in the BBC interview.

And that could be helpful, he said, as when Turkey mediated between Israel and Syria in 2008.

"I don't think, though, that this is at the expense of the West...We certainly haven't sensed any lessening of Turkey's desire to have a strong relationship with the United States and a strong relationship with Europe."

'Headache for the West'

Others see it differently, arguing that Ankara sees not only a leadership void in the Middle East but also weakened US influence.

Turkey has seen the leadership of the region up for grabs - and is going for itJosef Joffe, Editor, Die Zeit newspaper

"Next to Iran, Nato member Turkey is now the biggest headache for the West," wrote Josef Joffe, editor of the German weekly Die Zeit, in an opinion piece published in the Financial Times.

"With Egypt sinking into torpor and Riyadh firmly ensconced on the fence between Washington and Tehran, Turkey has seen the leadership of the region up for grabs - and is going for it," he added.

Part of that desire to lead has pushed Turkey to ride a wave of anti-Israel sentiment in the Arab world, starting with the Gaza offensive in 2008 and continuing with the Israeli raid against the Turkish vessel, the Mavi Marmara, on 31 May.

Washington has expressed concern about the tensions and said it was working to calm things down. While Turkish language towards Israel has been acerbic and threats have been made to reduce ties with Israel, no concrete moves have been taken yet.

Turkey's expectations

Mr Gordon said the US would work to preserve its relationship with Turkey though he also made clear the US had expectations.

"We hope and expect that Turkey will abide by the resolutions (imposing sanctions on Iran) as all other members of the UN are now required to do."

Turkey, too, has expectations. Turkey wants Washington to back its call for an international investigation into the flotilla incident.

Mr Yilmaz from the Seta think-thank warned that developments on this front would have an impact on Turkey's attitude towards the US in the future.